1. Field of the Invention
The present invention relates to a liquid crystal panel and a liquid crystal display apparatus which uses the liquid crystal panel.
2. Description of the Prior Art
The liquid crystal panel has been developed intensively because it has many advantages such as its light weight and compactness. However, there remain many problems. For example, it is difficult to enlarge the size of display area size. A projection type television has thus attracted attention wherein a display image of a small size liquid crystal panel is magnified by projecting it with a projection lens or the like on a screen. A commercial liquid crystal projection type television uses a twisted nematic liquid crystal panel which uses the optical rotation characteristic of liquid crystal.
A liquid crystal panel to be used in a projection type television has to be compact. Otherwise the size of projection lens increases and the system size also becomes large, and this results in a high cost and poor portability.
On the other hand, the number of pixels of a liquid crystal panel has had a tendency to be increased. For example, in the MUSE scheme of a highvision television of Japan, 1.3 million pixels are needed. If such a large number of pixels are formed in a compact liquid crystal panel, a ratio of the area required for switching elements for the pixels will increase. Usually, a thin film transistor is used as a switching element. In a commercial television set with a liquid crystal panel of a size of about three inches and a number of pixels of about 0.3 million, the aperture ratio is of the order of 30%. There is a demand that the aperture ratio be less than 10% if the number of pixels increases to one million or more in same panel size. Recently, the technology directed to thin film transistors in a liquid crystal panel has improved, and the problem of the aperture ratio is being solved by improving the precision of the mask design and process technology such as self alignment.
One of factors which affect the aperture ratio is the effect of the electric field due to source signal lines transmitting image signals to thin film transistors and due to gate signal lines transmitting signals to turn on or off the thin film transistors. The electric field changes the alignment of liquid crystal molecules in a twisted nematic type panel irrespective of the image signals, and a part of an image to be displayed vanishes around the periphery of pixels. Then, in order to prevent partial vanishing of the image, a shading pattern called as black matrix is provided on a substrate opposed to the other substrate on which pixel electrodes are formed. The shading pattern decreases the aperture ratio.
In order to solve problems related to the aperture ratio, a liquid crystal panel, with use of twisted nematic liquid crystal and reflection type pixels, and a projection type television with use of the panel have been fabricated (Proceedings of the 9-th International Display Research Conference (JAPAN DISPLAY '89) p584-587). In the liquid crystal panel, reflection electrodes are formed on thin film transistors, so that the area occupied by the thin film transistors is prevented from decreasing the aperture ratio due to the existence of the thin film transistors. Further, signal lines are formed below the reflection electrodes, so that the vanishing of a partial image due to the electric field of the signal lines can be prevented. Thus, the aperture ratio increases to 70% even though the pixel size is very small or 30.times.35 .mu.m.
However, this panel structure has problems. As explained above, the reflection electrodes are formed via an insulator film on the thin film transistors. Therefore, the pattern of the thin film transistors are transferred to the reflection electrodes, and the surface of the reflection electrodes becomes uneven even though the reflection electrodes are expected to have a specular surface. In order to prevent the unevenness, the thickness of the insulator film may be increased. However, such a thick film makes it difficult to form contact holes for electrical connection to the reflection electrodes. Further, the surface becomes very uneven at the contact holes. On the other hand, the thickness of the reflection electrodes may be increased. However, this increases the deposition time of a metallic film for the reflection electrode. Further, it becomes difficult to separate adjacent reflection electrodes on the patterning step. Another problem is with respect to the rubbing needed for the twisted nematic liquid crystal panel because a rubbing cloth is caught at peripheral parts of the reflection electrodes to make the alignment around the peripheral parts poor. Further, the rubbing pressure may break edges of the reflection electrodes and the generated fragments of the reflection electrodes may cause a short-circuit between reflection electrodes. Thus, the above-mentioned liquid crystal panel has an advantage of high aperture ratio, but it is difficult to display an image at a high luminance and the alignment of liquid crystal molecules are liable to become worse.
A twisted nematic type liquid crystal display also has a problem in that a polarized beam splitter is needed to construct a projection type television. The polarized beam splitter has a narrow angle range for incident light and its cost is high. Further, only one of P and S components can be used for a projection light. Thus, a projected image becomes darker.